In the paper-making process, chemical pulping yields, as a by-product, black liquor, which contains almost all of the inorganic cooking chemicals along with the lignin and other organic matter separated from the wood during pulping in a digester. The black liquor is burned in a recovery boiler. The two main functions of the recovery boiler are to recover the inorganic cooking chemicals used in the pulping process and to make use of the chemical energy in the organic portion of the black liquor to generate steam for a paper mill. The twin objectives of recovering both chemicals and energy make recovery boiler design and operation very complex.
In a kraft recovery boiler, superheaters are placed in the upper furnace in order to extract heat by radiation and convection from the furnace gases. Saturated steam enters the superheater section, and superheated steam exits at a controlled temperature. The superheater is constructed of an array of tube panels. The superheater surface is continually being fouled by ash that is being carried out of the furnace chamber. The amount of black liquor that can be burned in a kraft recovery boiler is often limited by the rate and extent of fouling on the surfaces of the superheater. This fouling reduces the heat absorbed from the liquor combustion, resulting in low exit steam temperatures from the superheaters and high gas temperatures entering the boiler. Boiler shutdown for cleaning is required when either the exit steam temperature is too low for use in downstream equipment or the temperature entering the boiler bank exceeds the melting temperature of the deposits, resulting in gas side pluggage of the boiler bank. Kraft recovery boilers are particularly prone to the problem of superheater fouling, due to the high quantity of ash in the fuel (typically more than 35%) and the low melting temperature of the ash.
There are three conventional methods of removing deposits from the superheaters in kraft recovery boilers, listed in increasing order of required down-time and decreasing order of frequency: 1) sootblowing; 2) chill-and-blow; and 3) waterwashing.
Sootblowing is the process of blowing ash deposit off the superheater with a blast of steam from nozzles called sootblowers. Sootblowing occurs essentially continuously during normal boiler operation, with different sootblowers turned on at different times. Sootblowing reduces boiler efficiency, since 5-10% of the boiler's steam is typically used for sootblowing. Each sootblowing operation reduces a portion of the nearby ash deposit, but the ash deposit nevertheless continues to build up over time. As the deposit grows, sootblowing becomes gradually less effective and results in impairment of the heat transfer.
When the ash deposit reaches a certain threshold where boiler efficiency is significantly reduced and sootblowing is insufficiently effective, deposits must be removed by the second cleaning process called "chill-and-blow"(also called "dry cleaning" because water is not used), requiring the partial or complete cessation of fuel firing in the boiler for typically 4-12 hours, but not complete boiler shutdown. During this time, the sootblowers continuously operate to cause the deposits to debond from the superheater sections and fall to the floor of the boiler. This procedure may be performed as often as every month, but the frequency can be reduced if the sootblowing is performed optimally (at the optimum schedule and in the optimum sequence). As with sootblowing, the chill-and-blow procedure reduces a portion of the nearby ash deposit, but the ash deposit nevertheless continues to grow over time. As the deposit grows, the chill-and-blow procedure becomes gradually less effective and must be performed more often.
The third cleaning process, waterwashing, entails complete boiler shutdown for typically two days, causing significant loss in pulping capacity at a mill. In a heavily fouled recovery boiler, it may be required every four months, but if the chill-and-blow process is properly timed (i.e. before large deposits form in the boiler bank section), then the shutdown and waterwashing can be avoided for even a year or longer.
In determining the optimum frequency, or time, to implement each cleaning process, there is a calculated tradeoff. Boiler deposits reduce pulping capacity through boiler efficiency, but removing those deposits through waterwashing temporarily reduces pulping capacity much more. Hence, there is an optimum frequency, or timing, for the waterwashing process, and doing it too often or too rarely is financially costly.
Similarly, there is an optimum frequency, or time, to implement the chill-and-blow cleaning process, based on the amount of deposit and the rate of fouling of the superheaters. Applying chill-and-blow too often unnecessarily increases down-time, and applying it too rarely increases the need for a complete shutdown and waterwashing. Therefore, more precision in timing the chill-and-blow process greatly increases boiler efficiency, with large financial and environmental benefits. A similar tradeoff applies to the sootblowing.
This tradeoff of economic considerations is discussed in U.S. Pat. No. 4,475,482, which describes a method for predicting the optimum cycle time to schedule sootblowing, based on economic criteria which account for heat transfer surface fouling, rate of fouling of other heat transfer surfaces within the boiler, and on-line boiler incremental steam cost.
The prior art methods of determining the amount of deposit on superheater sections of kraft recovery boilers, or the timing of cleaning, are based on indirect measurements, such as the temperature increase of gas exiting the boiler, the temperature decrease of steam, heat transfer, enthalpy, or the pressure drop increase over the gas side (combustion section as opposed to the water/steam side) of the boiler. The following patents disclose methods to assess the timing and efficacy of removing ash deposit by measuring factors affected by the deposit, but not by measuring the deposit weight. U.S. Pat. Nos. 4,454,840 and 4,539,840 disclose a method of identifying a parameter of a model for rate of loss of fossil fuel boiler efficiency due to a sootblowing operation, based on time since a last sootblowing in a heat transfer surface (convection-pass surface such as superheater and economizer) in question, overall boiler efficiency at the beginning of the sootblowing, and change in efficiency due to the sootblowing.
U.S. Pat. No. 4,718,376 discloses a method for controlling sootblowing in a chemical recovery boiler, entailing (column 4, line 27) instrumentation that indicates the change in heat transfer characteristics over time due to fouling, such as by measuring the change in flue gas temperature and pressure drop across the tube bank and change in enthalpy of water or steam in the tube bank. Alternatively, (column 4, line 47) the instrumentation is related to changes in boiler operating characteristics over time, such as steam rate, feed water rate, fuel firing rate or change in flue gas composition.
U.S Pat. No. 4,488,516 discloses a soot blower system for a fossil fuel fired steam generator comprising soot blowers selectively operable to clean ash from furnace chamber walls in direct response to the local heat transfer rate sensed by heat flux meters mounted to the furnace wall in the region surrounding each soot blower.
The prior art techniques each have one or more of the following problems: 1) They require the use of expensive and/or delicate equipment which can require recalibration. 2) They are affected by many boiler parameters (such as boiler load), and mathematical corrections for these interfering parameters are not precise. 3) They cannot be used when the boiler is partially or fully shut down for cleaning. 4) Some methods require complex formulas and parameters that are difficult for a common boiler operator to perform and understand.
The deposit weight, the optimum time for applying a cleaning process, and the effectiveness of the cleaning process displayed in real-time could be determined much more precisely if the weight of superheater deposit were measured directly.